The present invention relates to digital television, and more particularly, to processing that reduces quantizing noise.
In digital video, when too few bits are used to quantize each sample of the video waveform, quantization noise is large and causes objectionable contouring or "puddling" of the displayed picture. This is most objectionable in areas of gradual intensity changes where the slope of the video waveform is small compared to the quantization step size. This is because in such slowly changing, low frequency portions, of the picture, the eye is most sensitive to quantization noise. By increasing the number of bits per sample, more levels are available for quantization, and the gradual change is more closely represented. However, the use of more bits per sample results in the penalty of a corresponding increase in the required data rate of the digital video. It is known from the article entitled "PCM Encoded NTSC Color Television Subjective Tests" by A. A. Goldberg, JSMPTE, August, 1973, p.p. 649-654, to add either a square wave or a random signal to the video signal before quantization to reduce contouring, and then low pass filtering the reproduced analog video signal to reduce the visibility of the added signal and the quantizing noise. However in this system, the low pass filter also reduces the high frequency video signal information.
It is therefore desirable to reduce the visability of contouring, and quantizing noise without increasing the data rate and without reducing the high frequency video signal information.